Ubiquitin, Rashi say the opposite. Even if they were not written they would have been commanded. That is to say, people would prohibit them.
Doing, Rav Shimon Shkop addresses this in Shaarei Yosher. How do we allow a possessor to retain a disputed object where there is no proof? It is a safek d’Oraita of robbery and therefor we should be stringent and leave it aside until there is proof. He answers that before an object can be considered stolen the law must say that it does not belong to the possessor. The whole idea of the mitzvot is that there are actions which are inherently bad, in the case of the sheva mitzvot, or bad for us, in the case of the Taryag. Good and bad obviously were created before people as the Tree of Life was created before Adam and Chava.